This work is aimed at determining the cullular events in bone remodeling and the fate of nuclei in the osteoclasts, using alveolar bone and tooth migration as a model. Similar in vitro studies are also conducted with either tissue cultures or isolated bone cells. The techniques used are mainly dynamic bone histomorphometry on undecalcified thin sections with double fluorescent labels, electron microscopy and autoradiography. The time course of early effects of parathyroid hormone and calcitonin on the osteoclast population and the number of nuclei they contain have been studied during the first year of this project. During the current year, the effects of three diphosphonates Azacycloheptane-2,2-Diphosphonic acid (AHP), Dichloromethylene Diphosphonic acid (Cl2MDP) and 3-Amino-1-Hydroxy-Propane-1,1-Diphosphonic acid (APD) at concentrations ranging from 0.05 to 50.0 mg/Kg during 5 to 10 days of treatment have been studied. All three diphosphonates induced various degrees of inactivation of the osteoclasts associated with an increase in their number and an increase in the number of nuclei they contain. The total number of nuclei in the osteoclastic pool was therefore increased 3-fold. The effects of PTH on this accumulation of nuclei and then the effects of calcitonin will be studied next year in order to follow the fate of osteoclast nuclei. Of interest is the fact that tooth migration was maintained with a normal amount of bone matrix being formed despite a decrease in the calcification rate and in the resorption rate. In vitro studies on fetal rat long bones conducted this year indicated the need for having more homogeneous bone cell populations in order to separate the reactions of osteoclasts and osteoblasts to different agents. Studies using rabbit long bone fragments are planned in the coming year.